Polypropylbenzene, polybutylbenzene and polycyclohexylbenzene are significant by-products in the alkylation and hydroalkylation of benzene to produce cumene, butylbenzene, and cyclohexylbenzene respectively. As a result there is significant interest in processes for converting these poly-alkylated aromatic compounds into additional quantities of the desired mono-alkylated aromatic compounds.
Currently, the most common route for the conversion of poly-alkylated aromatic compounds to their monoalkylated counterparts is by transalkylation in which the poly-alkylated species is contacted with an excess of the base aromatic compound in the presence of a transalkylation catalyst. For example, U.S. Pat. No. 6,037,513 discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional hydroalkylation catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The '513 patent also discloses that any dicyclohexylbenzene in the reaction product can readily be separated from the product stream and converted to additional cyclohexylbenzene by transalkylation with the benzene feed, preferably using the same MCM-22 family molecular sieve as used in the hydroalkylation catalyst but in the absence of the metal components on the hydroalkylation catalyst and in the absence of a hydrogen co-feed.
However, although transalkylation is an effective method of converting poly-alkylated aromatic compounds into their mono-alkylated counterparts, it suffers from the disadvantage that a large excess of base aromatic compound, normally benzene, is required (typically ≧3:1 molar ratio of benzene: heavies), resulting in the need for significant benzene recycle streams. In addition, a low space velocity, and hence a large reactor, is normally required to achieve acceptable conversion in the transalkylation reaction.
According to the present invention, an alternative process for converting poly-alkylated aromatic compounds into additional mono-alkylated product is proposed in which the poly-alkylated aromatic compound is dealkylated, optionally in the presence of hydrogen and/or a small amount of benzene, to produce the mono-alkylated aromatic compound and an alkane or alkene with the same number of carbon atoms as the alkyl groups on the poly-alkylated aromatic compound.
U.S. Pat. No. 3,984,490 discloses a process for recovering phenylcyclohexane as principal product from dicyclohexylbenzenes by heating the latter in the presence of at least an equal amount by weight of benzene and an acid clay or zeolite catalyst at a temperature of 190° C.-400° C. The process in the '490 patent is described as “cracking” but the fact that the conversion requires “at least an equal amount by weight of benzene” suggests that the actual mechanism of the conversion is by transalkylation.